1. Field of the Invention
The present invention relates to a reactive hot melt adhesive composition (hereinafter, unless otherwise stated, will be referred to as “RHMA”), and a method for producing the same.
2. Description of the Related Art
A hot melt adhesive is an adhesive which is solid or semi-solid at room temperature, and melts when heated, to thereby obtain fluidity. This adhesive bonds an adherend to a base material, as the adhesive is applied on the base material in a molten state, and then the adherend is laminated thereon, or as the adhesive is inserted between the base material and the adherend in the form of a solid sheet, melted by heating, and then resolidified by cooling. Therefore, many hot melt adhesives are produced from thermoplastic polymers.
In particular, adhesives containing, as a main component, a urethane prepolymer having an isocyanate group at the chain terminals are called as a “reactive hot melt adhesive (RHMA)”, and most of such urethane adhesives are moisture-curable. The urethane adhesives usually contain, as a main component, an isocyanate group-having urethane prepolymer produced by polycondensation of a polyol component and a polyisocyanate component.
Such a RHMA is applied on a base material while being in a heated molten state, is cooled to solidify, and then undergoes moisture curing caused by a chemical cross-linking reaction of the isocyanate group subjecting with moisture (water), to thereby form a tough adhesive layer having heat and chemical resistances.
Conventional RHMAs exhibit excellent adhesiveness when chemical cross-linking is formed as a result of moisture curing. However, since most of the RHMAs do not undergo chemical cross-linking reaction immediately after being applied, and since the adhesives have high temperature and high fluidity immediately after being heated and melted, the initial adhesive strength to the base material is insufficient. Here, the initial adhesive strength means the adhesive strength that is obtainable 2 to 5 minutes after the adhesive is applied on a base material.
As a product which has solved the problem described above, a RHMA produced by heating and mixing of a polyisocyanate and an acrylic polymer having a hydroxyl group, is known (see WO2004/111102).
There is also known a RHMA produced by heating and mixing of a urethane prepolymer and an acrylic polymer having an epoxy group (see JP 2003-193019 A).
However, in regard to the former RHMA, the initial adhesive strength is not sufficient because the reactive functional group possessed by the acrylic polymer is only a hydroxy group, and the hot melt adhesives tend to have increased stringiness when melted and applied on a base material.
The term “stringiness” means that when a hot melt adhesive is applied on a base material with a heated nozzle, a sprayer, a coater or the like, fine strings are drawn between the adhesive applicator and the base material which serves as an adherend. Furthermore, increased stringiness implies that the adhesive sticks to the applicator or the manufacturing machinery, and handleability thereof is deteriorated.
On the other hand, in regard to the latter reactive hot melt adhesive, the initial adhesive strength is insufficient because the reactive functional group possessed by the acrylic polymer is only an epoxy group.